1. Field of the Invention
This invention relates to a brushless DC motor which does not have a position sensor for detecting a rotational position of a permanent magnet rotor.
2. Description of the Prior Art
Brushless DC motors have a longer service life and less noise generation for the reason that they do not need to have a mechanical contact as would be used in conventional DC motors having brushes. As a result, they have been widely used recently in industrial or audio and video equipment requiring higher reliability.
In order to perform the switching operation of a conducting phase of the stator windings of a motor, most of conventional brushless DC motors use a rotor position sensor (such as, for example, a Hall Effect sensor) instead of brushes. However, the rotor position sensor is not cheap, and it requires a sophisticated positional adjustment for setting and an increased amount of wirings, so that cost of the brushless DC motors is large as compared with DC motors having brushes. Besides, some structural limitations will be frequently imposed thereupon for the reason that a rotor position sensor has to be set inside the motor itself. A recent trend is that accompanied with the miniaturization of industrial or audio and video equipment, motors must be made small in size and thickness, which means that the sectional space where a motor position sensor, such as a Hall Effect sensor, is located becomes extremely small. Accordingly, several types of brushless DC motors having no position sensor, such as, for example, a Hall Effect sensor, have been previously proposed. Out of which, a known brushless DC motor uses an output pulse of a frequency generator fixed to a motor. This motor counts an output pulse of the frequency generator which generates a pulse in response to the rotation of the rotor by a counter circuit and outputs a preset driving current, having a current pattern in response to the count value thus obtained, to the three-phase stator windings of the rotor in a successive manner, thereby rotating the permanent magnet rotor (see, for example, Japanese Laid-Open Patent Application No. 63 -262088).
With the structure as explained above, the initial position of the rotor when starting cannot be found. As a result, the conventional brushless DC motor as shown above has a rest generating circuit for to resetting the counter using a reset signal when starting and for supplying a specific reset signal simultaneously to the stator windings of the rotor, whereby the rotor and the stator windings are placed in a predetermined positional relationship to each other in advance.
However, if the specific reset current is supplied to the stator windings in order to determine the initial position, the rotor starts to rotate and the position of the rotor vibrates around the predetermined position, so that it is impossible to stabilize the rotor at its predetermined position in a short period of time. As a result, it is impossible to shift the mode in a short period of time from the reset mode in which the specific reset current is supplied to the stator windings to stabilize the rotor to the predetermined position when starting to the normal position detecting mode in which an output pulse of the frequency generator is counted in response to the rotation of the rotor. Thus, there arises a problem in that the starting time becomes long.
This means that a conventional brushless DC motor cannot be used in an application in which the motor rotates and stops frequently and starts up in a short period of time.
In addition, with the conventional brushless DC motor as explained above, the initial position of the rotor is detected when starting, so that even if the rotor and stator windings are placed in a predetermined positional relationship to each other by supplying the reset current to the stator windings, when 1a load is applied to the rotor, the positional relationship between the rotor and stator windings will be largely varied depending on the magnitude of the load thus applied. As a result, it becomes impossible to stabilize the rotor at the predetermined position in the reset mode.
Accordingly, with the conventional brushless DC motor as shown above, if the mode is shifted from the reset mode to the normal position detection mode for counting the output pulse of the frequency generator in response to the rotation of the rotor, the phase of a current supplied to the stator windings is largely deviated from the normal phase, resulting in it being impossible to realize a highly efficient drive.
As a result, a conventional brushless DC motor is disadvantageous in that it can be used only in such an application that the motor itself is unloaded when starting.
In addition, with the brushless DC motor in which, as shown above, a output pulse of the frequency generator for generating a pulse in response to the rotation of the rotor is counted by the counter circuit and a driving current is output to the three-phase stator windings in a successive manner in response to the count value thus obtained so as to thereby rotate the permanent magnet rotor, the count value obtained by the counter circuit will be erroneous if the output of the frequency generator has noise overlapped for any reason during continuous driving, and such a count error thus generated leads to a reduction in efficiency of the motor and an increase in torque ripple. In addition, the accumulation of such count errors may cause the motor to stop in the worst case, which means that the conventional brushless DC motor as shown above lacks reliability.